Ceramic composition



United States Patent 2,805,165 CERAMIC COMPOSITION Gilbert Goodman,Niskayuna, N. Y., assignor' to General Electric Company, a corporationof New York No Drawing. Application April 25, 1955, Serial No. 503,818

21 Claims. (Cl. 106-39) My invention relates to polycrystalline ceramicmaterials and more particularly to ceramic materials consisting ofmodified lead metaniobate which are particularly useful as ferroelectricand dielectric materials. Polycrystalline dielectric materials whichhave been found to have electric polarization properties analogous tothe magnetic induction properties of ferromagnetic materials are nowgenerally known, as ferroelectric materials.

One property of these ferroelectric materials is that at a particulartemperature called the Curie point there is a definite transformation ofthe crystal structure. Ferroelectric ceramic materials heretofore known,such as alkaline earth titanates, have a crystalline structure identicalwith or similar to that of the mineral perovskite and undergo acrystalline structure transition such as, for example, thetransformation from a tetragonal to a cubic structure as the temperatureof the material is raised through its Curie point. Further, thesepreviously known materials have the type-form equation ABOa, in which ithas been postulated that the A ions may be selected from the first,second or third group of the periodic table, and the B ions from thesecond through sixth group. The ceramic materials of my invention differbasically from these materials in that the crystalline structure of mymaterials do not have the perovskite structure nor are they isomorphouswith the previously known perovskite materials. Further, the basicceramic material of my invention has the type-form equation A(BO3)2, inwhich the A ions are principally lead, which appears in group four ofthe periodic table, and the B ions are principally niobium.

Once polarized, ceramics of the previously known perovskite materialshave remanent piezoelectric properties only While they are maintained ina tetragonal or other crystalline state below this Curie temperature. 1'If ...-the Internal temperature of the polarized ferroelectric ceramicis raised above this Curie point, the piezoelectric property isdestroyed and the material must be reactivated I by thela'pplication ofa polarizing electric field before it will again exhibit remanentpiezoelectric properties in its inltial crystalline state below theCurie point. Unfortunately, the Curie point of ferroelectric perovskitecompounds rarely exceeds 120 C. (the Curie point of barium titanate).-The utility of these ceramics as piezoelectric elements in variouselectro-mechanical transducers is thus severely curtailed by this uppertemperature limitation. Where ceramic transducers are employed ascompressional wave generators, for example, the maximum power outputobtainable is often limited by the resultant temperature rise in thetransducer. Moreover, it is often desired to employ ceramic transducersto sense mechanical vibration in locations such as on jet enginesthathave temperatures far above 120' C. In my copending application,Serial No. 230,788, filed June 9, 1951, now Patent No. 2,729,757,andassign-ed to the assignee of the present application, I disclosepolycrystalline dielectric materials in the form of ceramics havingtransition tem- 2 peratures much higher thanperovskite ferroelectriccompounds. These ceramic materials are composed principally of firedlead metaniobate which may be fluxed with an oxide of a metal selectedfrom the group consisting of titanium and zirconium. The flux improvesthe mechanical strength and. density of ceramic bodies formed front leadmetaniobate and is therefore generally desirable. set forth in mycopending application, ceramic dielectric materials made in accordancetherewith have moderately high and fairly constant dielectric constantsover a wide temperature range fromnormal room temperature up totemperatures of the order of 540 C. 'Further, the meas-' ured daspiezoelectric constant of these lead metaniobates is in theneighborhood, of; 5. -10- coulombs per newton at room temperature. Y

In another copending application, Serial No. 449,507, filed August 12,1954, now Patent No. 2,731,419, and assigned to the assignee of thepresent application, I disclose polycrystalline dielectric materials inthe form of ceramics having transition temperatures much higher thanperovskite ferroelectric' compounds. These ceramic materials arecomposed principally of firedlead metaniobate containing a fluxcomprising an oxide of a metal selected from a first group consisting oftitanium and zirconium and an oxide of a metal selected from a secondgroup consisting of chromium and tungsten. As set forth in the lattercopending application, ceramic dielectric materials made in accordancetherewith havev moderately high and fairly constant dielectric constantsover a wide temperature range from room temperature up to temperaturesof the order of 500 C. Further,'the measured daa piezoelectric constantof these modified lead metaniobates is in the neighborhood of l0.1 10-coulombs per newton at room temperature.

In order to impartbulk piezoelectric properties to a ferroelectricceramic, the domains of its constituent crystals must be given a netcommon orientation. This is accomplished by subjecting the ceramic to anelectricv field gradient which at or near the Curie temperature can berelatively small. In the case of the high Curie temperature 570 C. and540 C., respectively, of the lead metaniobate ceramics disclosed in mytwo copending applications referred to above, however, Curie pointpolarization is not feasible because of the low resistivity of thematerial at or near the Curie point and it is necessary to carry out thepiezoelectric'activation at a temperature of the order of 250 C. In thislower temperature range, greater field strengths arere'quired toovercome the high coercive, force of lead metaniobate. At elevatedtemperatures of this order the electrical resistivity of the ceramicassumes importance since it' governs the voltage which a given sample ofmaterial will sustain. For ex:

ample, the polarizing voltages which may be applied to the ceramicsdisclosed in my copending application referred to previously are limitedto the order of 20 volts per mil at a temperature of the order of 250 C.and 70 volts per mil at a temperature of the order of 225 C.,respectively, beyond which field strengths dielectric breakdown occurs.Under these conditions, some, but not the maximum degree of activationmay be realized. Inasmuch as saturation polarization can most easily beachieved in a ferroelectric ceramic near the Curie temperature where thepolarizing electric field is assisted by the disordering influence ofthermal energy in overcoming the coercive field of the ferroelectric, itis desirable to lower the Curie temperatures of ceramic materials ofthis type to the point of optimum combination of highelements incapacitors and similar apparatus. Ideally,

in apparatus of this type, the dielectric constant of the dielectricshould not change in response to changes in temperature. However, smallchanges in dielectric constant with respect to. temperature variationsmay be tolerated, particularly if the relationship between thevariablesis "linear or approximately so. Under such conditions theapparatus or the circuit in which'it is to be used may be designed tocompensate for the predictable variation.

Accordingly, a principal object of my invention is to provide newdielectric materials, in the form of ferroelectric ceramics which mayhave transition temperatures ranging continuously from a Curie point ofabout 45 C. up to a temperature of the order of 550 C. -Another objectof my invention is to provide modified lead metaniobate ceramicmaterials which may be more com pletely polarized to exhibit higherremanent piezoelectric 4 The data set forth in the foregoing tableillustrates the relationship of the various constituents of a fewceramics prepared according to this concept which have been formed andfired, and their corresponding dielectric and piezoelectric properties.For example, the dielectric properties as a function of temperature showthat solid solution replacement of lead by barium takes placecontinuously up to about (Phase-Bangs) (NbO3)2, beyond which point asecond non-ferroelectric crystalline phase (probably Ba(NbOs)z) appears.Within the solid solution range the Curie point is lowered linearly from570 to about 375 C. Over this range of composition, i. e.

. up to about 13210.25, the Curie point dielectric constant is of theorder of about 6500. As the barium content is properties than has beenpreviously. A still further ob- I ject of my invention is to provide newpolarized ferroelectric ceramic compositions of the lead metaniobatetype havinghigher piezoelectric constant values than previouslyobtainable. I A yet further object of my invention is to providemodified lead metaniobate ceramic materials having a substantiallyconstant dielectric constant over substantial ranges of elevatedtemperatures.

In accordance with my invention I have produced nonperovskite ceramiccompositions of the lead metaniobate type which have excellentdielectric and ferroelectric properties and which may be activated toexhibit higher remanent piezoelectric properties than hithertobeforeknown for such compositions. The compositions of my invention comprisepolycrystalline aggregates formed by firing combinations of lead oxide,niobium pentoxide and an oxide of an element selected from the groupconsisting of magnesium, calcium, barium and strontium and mixturesthereof, having a generalized formula In this formula A represents anelement selected from the group consisting of magnesium, calcium, bariumand strontium and mixtures thereof. As indicated by the previously setforth generalized.formula, portions of the lead content of ceramic leadmetaniobate are replaced on a molar basis by equivalent quantities ofmagnesium, calcium, barium or strontium or mixtures thereof. Theimmediate effect of such substitutions is a lowering of the Curietemperature of the lead metaniobate from 570 C. It is believed that thislowering ofthe Curie point is the result of a distortion of the leadmetaniobate crystal lattice by the substitution of ions differingslightly from lead ions in size and polarizability.

Table I Curie Composition Ten p d k'" o Pb(Nbs)2 570 5.0 -350 (Pliers-0.05) s)2 530 12. 9 184 (PbOflO' DJU) (NbOa)- 163 gPboss' ue) (Nb0a)2.-- 219 Pbo.sa'B8o.2u)(Nb0a)2 135 (Phase-B 015) (Nb0s)2- 350(Photo-13 0.50)(Nb0s)z 375 190 (Pbnao-Sroso) (Nb0s)2- 475 3. 2 565(Films-Sims) (Nb0s)2- 370 16. 1 825 (Finns- D) (02- 257 O. 3 1, 250(Phone-S un) (Nb 03):. 165 18. 3 2, 900 o 45 S art) s)2 127 8. 4 3, 200(Pbo in TO.M)(N 3)2 105 1. 0 2,050 (Pbmo 030.10) (Nbos 2 510 630 (Pbosscur) (N b 3)2--- 460 5. 7 280 (PbD.75-C30.25)(Nb03)2 403 6 3 410 (Pbomr 00.30) (Nb0s)2 404 3. 9 310 (Plum-Mgr (NbOa) 552 150 (Phone-Mgr (Nb0a)z-550 100 (Plans-B 0 1!' 0.04)(N 3)2 469 6. 270 (Pbms Baa0t-Sl'0J1)(NbO3)2 487 6. 9 270 (Pboao-B onts-sroizs)(NbOa): 222 14. 7 1,830

increased beyond Batrzs, the Curie temperature remains constant at about375 C., but the Curie point dielectric constant progressively decreasesto, for example (Pbo.25.'Bao.75) (NbO3)2, about 1500. In all thesecompositions, however, the dielectric constant has a substantiallylinear variation with temperature from room temperature up to severalhundred degrees C. For example; the dielectric constant of(Pb0.75'Bao.25) (NbO3)2 varies along a substantially straight line fromabout 350 at room temperature to about 900 at 260 C. Similarly, thedielectric constant of. (Pbn.25'Bao.75)(NbO3)2 varies substantially as astraight line from about at room temperature to about 350 at 340 C. I a

In the ceramic compositions in which the lead content is partiallyreplaced by strontium, the Curie temperature is reduced substantiallylinearly from 570 C. for Pb(NbO3)2 to 475 C. for a composition of about(Pbo.s0-Sro.2o)(NbOs)2, which composition has a Curie point dielectricconstant of about 3500. As the strontium content is raised to SI0.25,SW35, Sro.5o, Sro.55 and Srosn, the corresponding Curie temperaturechanges to 440, 325, 127, 105 and the corresponding dielectric constantat these Curie temperatures is about 4000, 2700, 3150, 5000 and 3100,respectively. Compositions containing strontium in excess of State failto exhibit a Curie point and the dielectric constant assumes asubstantially flat, linear relationship to temperature variations. Forexample, the composition (Pbuzs-Srums) (NbOs)2 has a dielectric constantat room temperature of about 187 and increase of temperature up to about450. C. does not cause it to vary from this value by more than about 15,a small variation for such a wide temperature range.

The compositions containing calcium substituted for part of the leadbehave similarly to the barium-containing ceramics discussed above. Inthese materials, the progressive increase in calcium content to aboutresults in a linear'decrease in Curie temperature from 570 C. to about403 C., beyond which point the Curie temperature remains substantiallyconstant. Forexample, the dielectric constant at the Curie point fortrba-a-camnmbom is about 2500. V v

In the compositions containing magnesium substituted for a portion ofthe lead, the Curie temperature is lowered to about 552 C. by thesubstitution of magnesium for about 10 mol percent of the lead, i. e.

Pbm -Mg0.1o) (NbOs 2 replaced by mixtures of the elements of the groupconsisting of magnesium, calcium, strontium and barium ac cording to theformula (Pb1z'Aa:)(Nb03)2, in which A represents not only an element ofthe group but mixtures thereof and x represents a mol fraction, haveunusual and useful dielectric and piezoelectric properties. For example,in the series of compositions in which lead is partially replaced by amixture of barium and strontium, in which the ratio of barium tostrontium is 1:3, the Curie temperature decreases approximately linearlywith the increase in the mol percent of the mixture substitution. Forexample, the approximate composition (Pbono- Beams Smzzs) (NbOs)z has aCurie point of about 378 C. The composition listed last in the table hasa Curie point of 222 C., and the ceramic of this series havingapproximately the composition (Pbo.so-Bao.1'zs-Sro.52s) (Nb03)2 has aCurie point of about 45 C.

Similarly, in compositions in which a portion of the lead content isreplaced by barium plus strontium, in which the ratio of barium tostrontium is 1:1, the Curie temperature is reduced substantiallylinearly from 570 C. to about 160 C. for a composition having theapproximate formula of (Pbo.50'Bao.25'Sru.25)(NbO3)2 and to about 135 C.for a composition having the approximate formula(PbasoB21035'SI'0.35)(NbO3)2.

Compositions in which a portion of the lead content is replaced bybarium plus strontium, in which the ratio of barium to strontium is 3:1, the Curie point is reduced substantially linearly from 570 C. toabout 235 C. for a composition having the approximate formula (Pbom-Baoao' Sr0.10)-(NbO3)2 and to a Curie point of about 222 C. for acomposition having the approximate formula .From the foregoing it isapparent that by using element mixtures for A, it is possible to carrythe Curie point depression farther than is possible with any singleelement alone. Thus, the minimum Curie temperature possible in compoundsof the type PbI'Sr1-e(NbOs)2 is 105 C. for Pbo.40'S1'0.6o(NbO3)2. Asnoted above, however, (Pbo.30'Bao.1'75'Sr0.525)(NbO3)2 makes possible aCurie point of 45 C.

If desired, the density of these modified lead metani obate ceramics maybe increased by the addition of a flux and flux compensating elementsuch as for example, ZrOz as a flux and W03 to compensatefor thestructural defects introduced by the flux, as taught in my copendingapplication Serial No. 449,507, now Patent No. 2,731,419,

. referred to previously. One such typical fluxed composition has theapproximate formula -(Pb.o.s5 Bao.o4- Sro.11) [(Nboses- Zmoos 0312 andis analogous to a composition listed in Table I. This composition wasprepared from a mixture of raw materials having the following analysis:43.80 percent by weight PbSO4, 2.78 percent SrCOs, 1.74 percent BaC2O4,45.11

were similarly prepared and found to have room temperature dielectricconstants of 850, 2560 and 3630, respecspa ms tively. These. threebodies were found tohave das p1ezo-- electric constants of at 1east9.5.'10" 2.0X10-" and 6.0X10*- respectively.

It should be noted, however, that while a flux may be added to theceramics of my invention, that the modified ceramics of my invention maybe formed and fired to pro duce dense and quite strong bodies withoutthe use of such a flux.

The raw materials used in making these ceramics are preferably reagentgrade lead sulfate, high purity niobium pentoxide, and reagent grades ofbarium oxalate, strontium carbonate, calcium carbonate and magnesiumcarbonate.

The raw materials are prepared for firing: in accordance with the usualceramic techniques. The materials are first mixed together in a liquidsuch as acetone or amyl acetate in which the materials are not soluble.The mixture is then dried. and pulverized such as by ball-milling into afine powder so that the coarsest particles will pass a 200 mesh screen.The mixed oxide powder is then compacted into pellets for convenience inhandling and the reaction between the constituent oxides accomplished byfiring the pellets in an air furnace at about 1275 to r 1300". C. forabout one hour. The pellets of modified lead metaniobate are withdrawnfrom the furnace, cooled and recrushed to pass a 200 mesh screen. Theresultant powder is then formed into the desired shape and refired.Small elements or bodies may be formed by dry pressing the powder undera pressure of about 10 tons per square inch. The powder may also beadmixed with a suitable binder such as, for example, a solution ofpolyvinyl alcohol in water or the like and formed by pressing, extrudingor casting. The water introduced with the binding agent may then beremoved in any suitable fashion, for example, by heating the formedbodies at about 110 C. for several hours. The dry bodies are then firedin an air furnace. In order to prevent thermal stresses from causingphysical damage to the formed bodies it is preferred that the firingcycle start with the furnace at or near room temperature. Thetemperature is increased to about 1250 to 1400 C., depending oncomposition, over a period of about 5 to 7' hours. The bodies are heldat about 1250 to 1400 C. for about one hour and then permitted to coolin the furnace back to room temperature. Obviously, the time necessaryto fire these ceramic bodies will be dependent upon their size andshape. The ultimate firing temperature is important because little orinsufiicient sintering occurs below about 1250 C. and the compositionsmelt above about 1425- C.

The lead content is introduced as a sulfate to takeadvantage of the high(1000 C.) decomposition temperature of lead sulfate. The materials addedto modify the lead metaniobate, i. e. barium oxalate, strontiumcarbonate, calcium carbonate and magnesium carbonate, all decompose attemperatures below 1000 C. to form their corresponding oxides, BaO, SrO,CaO and MgO. The lead oxide (PbO) is thereby made available in a highlyreactive state for combination with the niobium pentoxide and the.modifying metal oxides. The rapid rise during thev first firingtemperature together with a highly reactive state of the lead oxide;produces a lead oxideniobium pentoxide-modifying metal oxide interactionwith little. opportunity for lead loss by running off or vaporization ofthe molten lead oxide. Only: about 1 to 2% of the theoretical lead.oxide. content is lost in the process. The firing is carried out in anoxidizing atmosphere, such as is provided by any air furnace, whichprevents the reduction of the modified lead metaniobate which may occurif there is an insufficiency of oxygen present.

After cooling the modified lead metaniobate ceramics may bepiezoelectrically activated by applying a unidirectional polarizingfield of from about 50 to volts per mil thickness for from about A: to 3hours. The temperature of the activation depends upon the Curietemperature of the particular composition. For ceramics .activationtemperature.

having Curie points above about 300 C., it hasbeen foundthataltemperature within about 200 C. to 275 C. is preferable. Formaterials with lower Curie temperatures than about 300 C., the materialsmay be activated at temperatures about to 50 below the Curie point usingthe above voltage gradients. Alternatively, the materialmaiy'be cooledthrough its Curie point while subjected to a voltage gradient of about10 to 30 volts per mil as is known in the art.

. In making a transducer it is preferable to activate these leadmetaniobate ceramics by applying the polarizing voltage to electrodespreviously deposited upon opposite major faces of the ceramic piece.Pure gold or silver deposited on the face; of the ceramic has been foundto make satisfactory electrodes. These activating electrodesthereaftenalsofunction as the electrodes for the trans ducer.,Commercial silvering preparations adapted to be fired onto the surfaceof the ceramic may be used to form such electrodes. In order to preventarcing, the polarization is'preferably carried out with the ceramicelement in an atmosphere of a dielectric gas at to 50 pounds per squareinch pressure while it is maintained at the Electronegative gases suchas, for example, sulfur dioxide, sulfur hexafiuoride,dichlorodifiuoromethane, decafluorobutane, octafluoropropane andtritiuorobromomethane, among others, are dielectric gases which aresuitable as a dielectric atmosphere during activation as disclosed in mycopending application Serial No. 449,507.

: From the foregoing itmay be seen that the present invention providesnew ceramic compositions having useful electrical properties; Forexample, ceramics may be madeaccording to my invention having anydesired Curie temperature from about 45 C. to 570 C. These materials areuseful, for example, as temperature sensing elements in controlapparatus since the change in electrical characteristics of a givenceramic composition occurs at a predeterminable temperature range. Inparticular, bariumiicontaininglead metaniobate ceramics containingfromiBaoloor'to Baa-2s are useful as piezoelectric materials fortransducer and capacitor applications. Compositions containing fromB8025 to Baoss are particularly useful as capacitor dielectrics becauseof their unusually flat dielectric constant characteristic oversubstantial temperature ranges.

Compositions containing strontium substituted in part for lead, aspointed out above are useful piezoelectric materials from Sruoot toSmso, and the compositions between Smzo' and Smso have unusually highdielectric constants at Curie points between about 475 to 105 C. Thecompositions containing more than Sraso are characterized by anunusually flat temperature-dielectric constant relationship. Further,the compositions containing from Sronm to Sumo are characterized by alinear reduction in Curie point with respect to composition change.

While useful ceramics of my invention may be prepared which containcalcium in partial substitution for lead up to Canes, the range Caoni toCaoas is characterized by a substantially linear relationship betweenthe amount of depression of .the Curie point and the mol percent calciumsubstituted.

The usual characteristics of the magnesium addition ceramics and themixed addition ceramics have been pointed out previously.

The above specific examples in composition have been set forth asillustrative of the invention, it being understood that variousmodifications can be made within the true scope and spirit of myinvention.

. 8 i What I claim, as new and desire to secure by Lett Patent of theUnited States -is:

1. A dielectric ceramic composition consisting essentially of a firedcombination of lead oxide, niobium pentoxide and an oxide of an elementselected from the group consisting of magnesium, calcium, strontium,barium and mixtures thereof, in such proportions that:v the compositionhas the approximate formula (Pb1-.zA:c) (NbOa)2', wherein A representsthe recited group and in which x has a minimum value of about 0.001 anda maximum.

value of about 0.85.

2. A ceramic composition as recited in claim 1. inv

which x has a maximum value of about 0.10 when A is magnesium, about0.25 when A is calcium, about 0.60

when A is strontium, about 025 when A is bariumv and about 0.85 when Ais a mixture of barium and 'strontiumi -3. A ceramic composition asrecited in claim 1 in which A is magnesium and x represents a value offrom" about 0.001 to 0.85. 4. A ceramic composition as recited in claim3' in which x represents a maximum value of about 0.10.

5. A ceramic composition as recited in claim 1 in which A is calcium andx represents a value of from about 6. A ceramic composition as recitedin claim 5 in- 8. A ceramic composition as recited in claim 7 in which xrepresents a maximum value of about 0.60.

9. A ceramic composition as recited in claim 8 in which x represents amaximum value of about 0.20.

10. A ceramic composition as recited in claim 7 in which x represents avalue from about 0.20 to about 0.60.

11. A ceramic composition as recited in claim 7 in which x represents avalue from about 0.60 to about 0.85. 12. A ceramic composition asrecited in claim 1 in which A is barium and x represents a value of fromabout 0.001 to about 0.85.

13. A ceramic composition as recited in claim 12 in which 2: representsa maximum value of about 0.25.

14. A ceramic composition as recited in claim 12 in which x represents avalue from about 0.25 to about 0.85.

15. A ceramic composition as recited in claim 1 in which A consists of amixture of barium and strontium and x represents a value of from about0.001 to about 0.85 16. A ceramic composition as recited in claim 15 inwhich A is a 3:1'ratio of barium and strontium and x represents a valueof from about 0.001 to about 0.85.

17. A ceramic composition as recited in claim 16 in which x has amaximum value of about 0.50.

18. A ceramic composition as recited in claim 15 in.

which A is a 1:1 ratio of barium and strontium and x represents a valueof from about 0.001 to about 0.85.

19. A ceramic composition as recited in claim 18 in which x represents amaximum value of about 0.70.

20. A ceramic composition as recited in claim 15 in which A is a 1:3ratio of barium and strontium and x represents a value from about 0.001to about 0.85.

21. A ceramic composition as recited in claim'20 in which at representsa maximum value of about 0.70.

References Cited in the file of this patent UNITED STATES PATENTS2,584,324 Bouslry Feb. 5, 1952 'gaptri

1. A DIELECTRIC CERAMIC COMPOSITION CONSISTING ESSENTIALLY OF A FIREDCOMBINATION OF LEAD OXIDE, NIOBIUM PENTOXIDE AND AN OXIDE OF AN ELEMENTSELECTED FROM THE GROUP CONSISTING OF MAGNESIUM, CALCIUM, STRONTIUM,BARIUM AND MIXTURES THEREOF, IN SUCH PROPORTIONS THAT THE COMPOSITIONHAS THE APPROXIMATE FORMULA (PB1-X''AR)(NBO3)2, WHEREIN A REPRESENTS THERECITED GROUP AND IN WHICH X HAS A MAXIMUM VALUE OF ABOUT 0.001 AND AMAXIMUM VALUE OF ABOUT 0.85.